The growth in use of small electrically-powered devices has increased the demand for very small metal-air electrochemical cells. Metal-air cells have gained significant popularity because only the anode reaction material need be packaged in the cell, while the cathode reaction material is oxygen, which is drawn from the surrounding environment.
Such small cells are usually disc-like or pellet-like in appearance, and are about the size of garment buttons. These cells generally have diameters ranging from less than 6.4 millimeters to about 25 millimeters, and height ranging from less than 2.1 millimeters up to about 15 millimeters. The small size and the limited amount of electrochemically reactive material which can be contained in such small metal-air cells result in considerable attention being directed to improving the efficiency and completeness of the electrochemical reactions, which are used in such cells for generating electrical energy.
Metal-air cells take in atmospheric oxygen, and convert the oxygen to hydroxyl ions in the air cathode. The hydroxyl ions then migrate to the anode, where they cause the metal contained in the anode to oxidize. Usually the active anode material in such cells comprises zinc.
More particularly, the desired reaction in a metal-air cell air cathode involves the reduction of oxygen, the consumption of electrons, and the production of hydroxyl ions, the hydroxyl ions being able to migrate through the electrolyte toward the anode, where oxidation may occur, forming zinc oxide.
In most metal-air cells, air enters the cell through a port in the bottom of the cathode can. The port extends through the bottom the cathode can, and may be immediately adjacent the cathode assembly, or may be separated from the cathode assembly by an air reservoir or an air diffusion member.
In any of such arrangements, the port facilitates the movement of air through the bottom of the cathode can and to the cathode assembly. At the cathode assembly, the oxygen in the air reacts with water as a chemically reactive participant in the electrochemical reaction of the cell, and thereby forms the hydroxyl ions.
Since the overall electrochemical capacity of any electrochemical cell is to some extent determined by the quantity of electrochemically reactive materials which can be loaded into the cell, it is important to maximize the size of the cavity, in the cell, which is devoted to containing the reactive materials. In the case of a metal-air cell, contained reactive material is limited to the anode material. The improvements recited herein could, however, be applied to a variety of other electrochemical cells, and need not be, indeed are not, limited to metal-air cells.
In general, the size of any given cell is limited by the inside dimensions of the space provided in the article, or appliance, in which the cell will operate. For example, the size of a hearing aid cell is limited to the internal dimensions of the space, provided for the cell, in the hearing aid appliance. The internal dimensions of the space are determined by the hearing aid manufacturer, not the power cell manufacturer.
Thus, any given appliance includes a limited amount of gross space/volume allotted to occupancy by the electrochemical cell which powers the appliance. That gross space may ultimately be divided according to four competing functions. A first and minimal portion of the space is used to provide clearance between the interior elements of the space and the exterior elements of the electrochemical cell.
A second portion of the space is occupied by the structural and otherwise non-reactive elements of the electrochemical cell.
The third portion of the space is occupied by the electrochemically reactive materials of the electrochemical cell, and especially the anode material.
Finally, a fourth portion of the space, if used, can sometimes be described as "wasted" space, because it serves none of the above first through third portions. Such "wasted" space is typically found outside the cell, e.g. at corner locations, where the corner of the cell is less "square" than is structurally feasible, thereby wasting volume that potentially might be occupied, either directly or indirectly, by electrochemically reactive material. Such "wasted" space might also be considered to be included in the space allocated to "clearance" because such space is typically located outside the cell.
Any increase in the third portion of the space, namely the cavity in the anode can which cavity is allocated to the anode material, is necessarily gained at the expense of one or more of the other three portions of the fixed volume allocated for occupation by the cell, namely the first clearance portion, the second portion devoted to the non-reactive elements of the cell, or any fourth waste portion. Thus, it is important to identify the first, second, and fourth portions of the overall space, and to reduce the absolute amount of the space devoted to such uses. To the extent such uses can be reduced, the space so recovered can, in general, be allocated for use to hold additional amounts of electrochemically reactive anode material, thereby increasing the potential overall capacity of the cell to generate electrical energy.
Of the first, second, and fourth portions of the cell, the first portion, devoted to clearance, appears to hold the least potential for providing much if any significant opportunities for reduction in volume, and the total volume of the "clearance" space is typically relatively small. Namely, overall cell height and width dimensions are specified by the International Electrochemical Commission (IEC). While some manufacturers may employ designs which utilize readily definable wasted space on the outside of the cell, especially at lower corners of the cell, other manufacturers appear to more fully utilize the allocated space.
Accordingly, while some small amount of volume may be recovered from "wasted" space, or by reducing the "clearance" space, applicants have concluded that the greatest potential for recovering space for use in holding anode material, and thus to increase "volume efficiency" of the cell, lies in the second portion of the cell, namely the structural and otherwise non-reactive elements of the cell. These elements generally comprise the cathode can, the anode can, the seal, and the cathode assembly, these typically being all of the major structural elements of the cell except for the reactive anode material. Thus, to get more space for holding the reactive anode material, that space must generally be taken away from the anode can, the cathode can, the cathode assembly, or the seal, or some combination of these.
It is an object of this invention to provide electrochemical cells having increased fractions of the cell-receiving space devoted to containing electrochemically reactive anode material.
It is another object to provide metal-air electrochemical cells wherein the thicknesses of one or more of the anode can, the cathode can, the cathode assembly, or the seal have been reduced to thicknesses never before achievable for an electrochemical cell.
It is still another object of the invention to provide such cells wherein the structural integrity of the cell, to abuse from forces outside the cell, is maintained.
Yet another object is to provide improved anode cans, especially with reduced thicknesses.
Still another object is to provide improved cathode cans, especially with reduced thicknesses.